Method and apparatus for drying objects using aerosols

ABSTRACT

Method and apparatus for drying objects that may have been wetted in a manufacturing The objects are submerged in a rinse liquid in an enclosed chamber, and aerosol particles from a selected drying liquid are introduced into the chamber above the rinse liquid surface, forming a thin film on this surface. As the rinse liquid is slowly drained, some aerosol particles settle onto and form a film on the exposed surfaces of the objects, and displace and remove rinse liquid residues from the exposed surfaces. Surface contaminants are also removed by this process.

FIELD OF THE INVENTION

This is a continuation in part application of "Method and Apparatus forDrying Parts and Microelectronic Components, Including IntegratedCircuit Wafers, Using Sonic Created Mist," U.S. Ser. No. 08/484,921,filed Jun. 7, 1995, by the same inventor. This invention relates todrying of manufactured objects, including electronic components, usingaerosols created by sonic or ultrasonic means.

BACKGROUND OF THE INVENTION

Objects that are being manufactured using processes involvingapplication of liquids and other fluids often require that the parts bethoroughly dried before the manufacturing process can continue. Forexample, in fabrication of integrated circuits, doping, photomasking,etching and passivation processes often require application ofparticular liquids at one stage and removal of liquid residues beforethe next stage proceeds. Drying and removal of these liquid residuesmust be complete, but the drying process should, ideally, occur in arelatively short time interval and with expenditure of a minimum ofenergy and chemicals to implement the drying process.

Several workers have disclosed methods for drying parts, includingintegrated circuits, by use of heated or superheated gases. McConnell etal, in U.S. Pat. Nos. 5,577,650, 4,633,983, 4,738,272, 4,778,532,4,856,844, 4,899,767, 4,911,761, 4,917,123 and 4,984,597, disclosemethods of drying semiconductor wafers by flowing a heated vapor orfluid past the wafers to be dried in a vessel, as part of a waferprocessing sequence. The preferred drying vapor is superheatedisopropanol, which forms a minimum boiling azeotrope with water and isbelieved to displace water from the wafer surfaces, and the vapor flowsinto the vessel at one end and simultaneously flows out of the vessel atanother end.

In U.S. Pat. No. 5,383,484, Thomas et al disclose use of a plurality ofmegasonic beam transducers, located at staggered positions, for cleaningwafers. Each transducer emits a vibratory megasonic beam with anunspecified (very high) frequency in a fixed direction, and thetransducer locations are chosen so that the collection of beamsirradiate, and thereby clean, all wafer surfaces in a chamber, no matterhow the wafers are arranged.

Use of ultrasonic transducers in a chemical cleaning bath tocooperatively remove contaminants and unwanted material layers fromsemiconductor wafers, medical instruments and other objects of interestis disclosed by Erickson et al in U.S. Pat. No. 5,178,173, by Watanabeet al in U.S. Pat. No. 5,203,798, by Tamaki et al in U.S. Pat. No.5,227,001, by Evans et al in U.S. Pat. No. 5,248,456, by Smith et al inU.S. Pat. No. 5,337,446, by Koretsky et al in U.S. Pat. No. 5,368,054,by Steinhauser et al in U.S. Pat. No. 5,380,369, by Shibano in U.S. Pat.No. 5,447,171, by Awad in U.S. Pat. No. 5,464,477, by Kato in U.S. Pat.No. 5,467,791, by Thjietje in U.S. Pat. No. 5,468,302 and by Campbell inU.S. Pat. No. 5,472,005.

Use of ultrasonic transducers to coat, spray, deposit or otherwise applya desired material to an object surface is disclosed by Bachmann in U.S.Pat. No. 5,387,444, by Erickson et al in U.S. Pat. No. 5,409,163, and byVersteeg et al in U.S. Pat. No. 5,451,260. An ultrasonic fogging deviceis disclosed by Munk in U.S. Pat. No. 5,454,518.

These approaches use heated or superheated gases or direct beamirradiation to dry an object surface; or they use cooperative action byan ultrasonic beam and an active chemical bath to remove contaminantsfrom, or to apply a desired material to, an object surface. Theseapproaches are complex, usually require operation at high temperatures,often require processing times of several minutes, and often require useof specially resistant chamber walls for the processing chamber.

What is needed is a method and associated apparatus for drying objectsin a manufacturing process that works well at room temperature and issimple, that is demonstrably complete, with no significant residues,that can be accomplished in times as short as one minute, that can beperformed in a chamber with chamber walls made of almost any material,and that requires use of only a very small amount of a drying agent,with minimal expenditure of energy, particularly thermal energy.Preferably, the process should be performable over a wide range oftemperatures, and should be easily scalable to any size surface.

SUMMARY OF THE INVENTION

The needs are met by the invention, which provides a method andassociated apparatus for drying objects by use of recyclable water and asmall amount of a low surface tension liquid plus (optionally) briefapplication of a recyclable cleaning agent. In one embodiment, theobjects to be dried are submerged or immersed in a rinse liquid, such aswater, in a chamber. The rinse liquid surface is covered with a verythin film of a low surface tension selected liquid, such as isopropylalcohol ("IPA"), formed from an aerosol created by sonic or ultrasonicvibrations of a small stream of the selected liquid. Other suitableliquids include ethyl alcohol, methyl alcohol, tetrahydrofuran, acetone,perfluorohexane, hexane and ether. The thin film is continuallyreplenished as needed, and the rinse liquid covering the objects to bedried is slowly drained. As the rinse liquid and thin film drain fromthe object surface(s), the selected liquid briefly contacts the surfacesof the objects and removes water residues by a "chemical squeegeeing"process that is discussed later. Optionally, the objects can besubjected to an additional chamber purge or drying process, using aheated or ambient temperature cleaning fluid, such as dry N₂ CO or CO₂gas, after the chamber has been drained.

Process parameters that can be varied to control the process includevibration frequency for creation of aerosol particles from the selectedliquid, a representative aerosol particle diameter, delivery rate forthe selected liquid, pressure and temperature at which the selectedliquid is delivered for creation of the aerosol particles, temperatureof the drying fluid used (if any), and choice of the selected liquid andof the drying fluid used (if any).

The invention requires as little as 1-2 milliliters (ml) of the selectedliquid to dry objects in a chamber with volume of 10-20 liters, orsmaller or larger, if desired. This approach provides several benefits.First, the process is carried out at or near room temperature, withlittle energy expenditure, and does not require use of heated orsuperheated liquids or gases for drying. Second, the process uses a verysmall amount of the selected liquid in a large volume of rinse liquid(10-20 liters) so that the mixture of rinse liquid and selected liquidcan normally be disposed of without the special handling proceduresrequired for hazardous materials. Third, a wide variety of inexpensiveselected liquids can be used. Fourth, use of a covering film of selectedliquid minimizes vapor from the rinse liquid remaining in the chamberafter drainage. Fifth, the process is easily scaled up or down, with nosubstantial changes in the apparatus. Sixth, the process removes largediameter contaminants that are not chemically bound to an objectsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates suitable apparatus, in one embodiment, for practisingthe invention, with the objects submerged in a rinse liquid in achamber.

FIGS. 2A and 2B are schematic views of aerosol creating vibrating headsand nozzles suitable for use with the invention.

FIG. 3 illustrates the apparatus of FIG. 1 with the rinse liquid partlydrained from the chamber.

FIG. 4 is a flow chart of one embodiment of the method.

DESCRIPTION OF BEST MODES OF THE INVENTION

FIG. 1 illustrates one embodiment of apparatus 10 that is useful forpractising the invention. An enclosed chamber 11 is defined by a housing12 and is provided with a rack (optional) for holding the objects 13A,13B, 13C, etc. to be dried. The objects 13A, 13B, 13C are placed into,and removed from, the chamber 11 through a slidable, hinged or otheroperable entryway 15 that is part of the housing 12. When the entryway15 is closed or engaged, the chamber is enclosed, preferably in angas-tight manner, and any remaining gas in the chamber can optionally beremoved. A first port 21 and associated first valve 23 are attached tothe housing 12 and are connected to a source 25 of water or othersuitable rinse liquid 27 in which the objects 13A, 13B, 13C areinitially submerged. A second port 31 and associated second valve 33 areattached to the housing 12 and are connected to a selected liquid source35, such as a pressurized tank maintained at a pressure of 5-50 psi, ofa selected drying liquid or fluid 37 ("selected liquid") that willprimarily dry the objects 13A, 13B, 13C.

A third port 41 and associated third valve 43, which may coincide withthe first port 21 and first valve 23, are attached to the housing 12 andare connected to a first liquid or fluid tank or other suitable firstdrain acceptor 45 that receives and drains the rinse liquid 27 andabsorbed selected liquid 37 from the chamber 11. A fourth port 51 andassociated fourth valve 53, which may coincide with the second port 31and second valve 33, are attached to the housing 12 and are connected toa second liquid or fluid tank or other suitable second drain means 55that receives and drains the selected liquid 37, and aerosol droplets 39from the selected liquid, from the chamber 11.

Initially, the objects 13A, 13B, 13C are placed in the chamber 11 in arack or cassette (not shown), the entryway 15 is closed or engaged, thechamber is evacuated, and rinse liquid 27 is admitted to the chamberthrough the first port 21 and first valve 23 so that the objects arefully submerged or immersed in the rinse liquid. The first valve 23 isthen closed. Alternatively, the objects 13A, 13B, 13C may be partlysubmerged or immersed in the rinse liquid 27 so that a portion of thesurfaces of these objects are exposed above the exposed surface of therinse liquid.

A small stream of the selected liquid 37 then passes through the secondport 31 and second valve 33 and is received by a piezoelectricallydriven head 61 and vibrating sonic or ultrasonic nozzle 63 that vibratesat a selected frequency flying in the range 10 kHz≦f≦1000 kHz, and morepreferably in the narrower range 20 kHz≦f≦100 kHz. The driven head 61 isconnected to and driven by a frequency generator 64 that is preferablylocated outside the chamber 11 and that permits selection of a vibrationfrequency f in the indicated range. When the selected liquid 37 ispresent in the vibrating head 61 and vibrating nozzle 63 and the nozzleis vibrating, the selected liquid is converted into a plurality ofaerosol droplets 39 that move into the chamber 11 and occupy most or allof an upper portion 11U of the chamber that is not already filled by therinse liquid 27 and submerged objects 13A, 13B, 13C.

FIG. 2A illustrates a suitable drive head 61A and vibrating nozzle 63Athat can be used with the apparatus shown in FIG. 1. The vibratingnozzle 63A preferably has a hollow column 65A formed therein withdiameter d(col)≈200 μm, through which the selected liquid 37(cross-hatched) flows. The vibrating nozzle then "shakes off" smalldroplets 39 of selected liquid 37, which form aerosol droplets in aroughly cylindrical pattern and move into the portion of the chamber 11above the rinse liquid.

FIG. 2B illustrates another suitable drive head 61B and vibrating nozzle63B, including a thin hollow column 65B therein through which theselected liquid 37 flows. A housing 67B surrounds the nozzle 63B anddirects a ring of hot or cold inert gas 69B toward the aerosol droplets39, which move into the chamber in a conical or other desired patternfor enhanced distribution of the aerosol droplets throughout thechamber.

I have found that use of a higher frequency f will tend to produceaerosol droplets 39 with a smaller mean diameter d(mean). For avibration frequency f in the range 20 kHz≦f≦100 kHz, I estimate that themean aerosol droplet diameter lies in the range 10 μm≦d(mean)≦50 μm. Themean droplet diameter can be varied by varying the diameter(s) d(mem) ofthe membrane apertures 66 and by varying the frequency f of vibration ofthe vibrating nozzle 63A or 63B.

The selected liquid 37 should be non-reactive with the objects 13A, 13B,13C and with the walls of the chamber 11 and should have a substantiallylower surface tension than the surface tension of the rinse liquid.Suitable selected liquids include isopropyl alcohol, ethyl alcohol,methyl alcohol, tetrahydrofuran, acetone, perfluorohexane, hexane andether, as well as many other low surface tension liquids and fluids. Useof any of these substances as a selected liquid does not requireprovision of chamber walls made of specially-resistant materials.

The selected liquid 37 may be held in the selected liquid source 35 at apressure of 5-50 psi above atmospheric pressure to facilitate deliveryand to suppress the slight volatilization of the selected liquid thatmight otherwise naturally occur. The preferred rinse liquid, de-ionizedwater, has a surface tension σ=73 dynes/cm at T≈20° C., and organicmolecules such as methyl alcohol, ethyl alcohol, isopropyl alcohol,n-hexane and ether have surface tensions σ in the range 17 dynes/cm≦σ≦23dynes/cm at T=20° C. so that σ(selected liquid)<<σ(rinse liquid) at roomtemperature.

Use of the selected liquid 37 at or near room temperature is preferredhere. Use of the selected liquid 37 at a substantially elevatedtemperature can reduce the surface tension of the rinse liquid 27,relative to the surface tension of the selected liquid 37, and thusinterfere with the chemical squeegee effect relied upon for thisprocess.

An aerosol particle is a cluster or collection of molecules of theselected liquid 37 that has not undergone a phase transformation into avapor form. Thus, the energy E(aerosol) (1.6 Watts for a typical sonichead, or less than 100 Joules/gm at a flow rate of 2 ml/min) required toconvert one gram of the selected liquid 37 into aerosol droplets 39,provided by the vibrating nozzle, is much less than the energy ofvaporization E(vapor) required to heat and convert one gram of theselected liquid 37 into its vapor form. I estimate that the ratioE(aerosol)/E(vapor) is less than 2 percent. Production of the aerosolparticles can be carried out at or near room temperature, and use of avery high temperature, such as T=60-200° C., is neither needed noradvisable for this process. Further, only a small amount of the selectedliquid 37, as low as 1-5 ml, is required for drying several objects 13A,13B, 13C in the chamber 11.

The aerosol droplets 39 move into the chamber 11, and many of thesedroplets settle onto an exposed surface 29 (preferably calm) of therinse liquid 27 as a thin film 30 having a varying thickness h(aerosol).An estimated time required to form this thin film 30 is 40-60 sec. Aportion of the aerosol droplets 39 that join the film 30 will diffuseinto the rinse liquid 27 so that, if this film is not replenished withadditional aerosol droplets, the film 30 will quickly and substantiallydisappear. Preferably, the volume flow rate r(sel) of the selectedliquid 37 to the vibrating nozzle 63 is adjusted so that the rate atwhich aerosol droplets 39 join the film 30 is sufficient to maintain orincrease a selected thickness h(aerosol) for the film. A preferred rangefor the film thickness h(aerosol) is 0.5 mm≦h(aerosol)≦5 mm, but thisthickness may be made larger by increasing the volume flow rate r(sel).For a chamber 11 having an exposed (upper) surface for the rinse liquid27 with an area of about 900 cm², a volume flow rate r(sel)=r2 =1-5 mlper minute of the selected liquid 37 suffices. Usually, a volume flowrate r2=1-2 ml/min is high enough. The time required to drain thechamber at a drain rate of 5 mm/sec is about 20-40 sec for asemiconductor wafer 10-20 cm in diameter. Thus, very little of theselected liquid 37 is absorbed or diffuses into the rinse liquid 27 inthe course of the time interval (60-100 sec) required for establishmentof the film and draining of the chamber.

Because so little of the selected liquid 37 is used in the process, theselected liquid source 35 may have a relatively small volume, as littleas 20-25 ml, and the selected liquid source 35 may be located at aconsiderable distance, such as 1-4 meters, from the chamber 11. Thisenhances the safety of the process, where a selected liquid is used thathas a low flash point or that can initiate an explosion.

A very small amount of the selected liquid 37 will vaporize naturally atthe process temperature, preferably room temperature, based on theequilibrium vapor pressure coefficient of the selected liquid at thattemperature. This vaporized portion should be relatively small in theenclosed chamber 11 at room temperature, and the vapor portion of theselected liquid 37 will quickly come to equilibrium with the liquid filmand aerosol portions of the selected liquid 37. Use of a processtemperature much higher than room temperature would produce a selectedliquid 37 with a moderately higher equilibrium vapor pressurecoefficient and a comcomitantly higher amount of vapor from the selectedliquid. This natural vaporization of a small part of the selected liquid37 is not regarded as a useful part of the drying process.

After a film 30 of the aerosol droplets is established on the surface 29of the rinse liquid 27, which may require 40-60 sec, the rinse liquid 27is slowly drained from the chamber 11 through the third port 41 andthird valve 43 into the drain tank 45. Draining of the rinse liquid 27will require an estimated 20-40 sec for a chamber holding 10-20 litersof the rinse liquid 27. A preferred range for the drain rate r(drain) is3-10 mm/sec decrease in the height of the rinse liquid 27 in the chamber11, and r(drain)=5 mm/sec is a suitable drain rate for this process.Draining occurs slowly in order to preserve the thin film 30 of theselected liquid 37 at the otherwise-exposed surface 29 of the rinseliquid. As draining of the rinse liquid 27 proceeds, aerosol droplets 39continue to be produced by flow of a small stream of the selected liquid37 through the vibrating nozzle 63. The volume flow rate r(sel) of theselected liquid 37 may be adjusted toward higher or lowers values asdraining of the rinse liquid 27 (and absorbed aerosol particles 39)proceeds.

As the rinse liquid 27 drains from the chamber 11, the surfaces 14A,14B, 14C of the objects 13A, 13B, 13C are increasingly exposed above theexposed rinse liquid surface 29 and overlying film 30, and aerosoldroplets 39 in the upper part of the chamber 11U settle onto theseexposed surfaces 14A, 14B, 14C, as shown in FIG. 3. Also, a portion ofthe film 30 of the selected liquid 37 may settle on the exposed portionsof the object surfaces 14A, 14B, 14C, rather than moving with the rinseliquid 27 toward the third port 41. The selected liquid 37 is chosen tohave a much smaller surface tension σ(sel) than the surface tensionσ(rinse) of the rinse liquid 27. If the rinse liquid 27 is water, theassociated surface tension is σ(rinse)=73 dynes/cm at room temperature.In this instance, the selected liquid 37 may be isopropyl alcohol("IPA") or ethyl alcohol or methyl alcohol, with the respective surfacetensions of σ=21.7 dynes/cm, 22.6 dynes/cm, and 22.8 dynes/cm at roomtemperature. The selected liquid 37 is also chosen for its ability todisplace rinse liquid at whatever process temperature is used. Roomtemperature (T=20° C.), and even lower temperatures, can be used here.The process also works satisfactorily at somewhat higher temperatures.

As exposed portions of the object surfaces 14A, 14B, 14C receive theaerosol droplets 39 of the selected liquid 37, new films 16A, 16B, 16Cof the aerosol droplets 39 or selected liquid 37 form on these exposedportions. As draining of the rinse liquid 27 from the chamber 11proceeds, and after draining is completed, the selected liquid 37 in thefilms 16A, 16B, 16C displaces most or all of the rinse liquid 27 thatremains on the exposed portions of the object surfaces 14A, 14B, 14C, inlarge part because the surface tension σ(sel) of the selected liquid 37is much smaller than the surface tension σ(rinse) of the rinse liquid27. The rinse liquid 27 that is displaced by the selected liquid runsdown the exposed surfaces 14A, 14B, 14C of the objects 13A, 13B, 13C andis drained away with the bulk of the rinse liquid in the chamber. Theselected liquid 37 that forms a film on the surfaces 14A, 14B, 14C ofthe objects 13A, 13B, 13C also runs down these surfaces and is drainedaway with the bulk of the rinse liquid 27. The films 16A, 16B, 16C ofselected liquid 37 thus act as "chemical squeegees" in removing rinseliquid 27 and selected liquid 37 from the exposed surfaces 14A, 14B, 14Cof the objects 13A, 13B, 13C.

This chemical squeegeeing of the objects' exposed surfaces 14A, 14B, 14Chas another benefit. The process not only dries the objects' surfacesbut also removes most of the larger contaminant particles from thesesurfaces, if these contaminant particles are not chemically bound to thehost surfaces. I have examined some bare silicon surfaces before thechemical squeegeeing process is applied and have found a substantialnumber of contaminant particles with diameter at least 0.3 μm on thesesurfaces, as indicated in column (2) of Table 1. I have then applied thechemical squeegeeing process discussed here, have re-examined the samesurfaces after completion of the chemical squeegeeing process, and havefound the number of contaminant particles is reduced after completion ofthe chemical squeegeeing process, as shown in column (3) of Table 1.These results indicate that chemical squeegeeing alone removes 12-100percent of the contaminant particles with diameters greater than 0.3 μm,depending on size.

                  TABLE 1                                                         ______________________________________                                        Chemical Squeegee Removal of Large Contaminant Particles                                   Particles before                                                                          Particles after                                      Particle Size                                                                              Chem. Squeegee                                                                            Chem Squeegee                                        ______________________________________                                        0.329-0.517 μm                                                                          8           7                                                    0.518-0.810  7           2                                                    0.811-1.270  7           2                                                    1.271-1.990  3           1                                                    1.991-3.130  6           1                                                    3.131-4.910  6           0                                                    ______________________________________                                    

At about the time the rinse liquid 27 becomes fully drained from thechamber 11 and the surfaces 14A, 14B, 14C of the objects 13A, 13B, 13Care fully exposed, the second port 31 and second valve 33 are closed,the vibrating nozzle 63 is shut down, and the fourth port 51 and fourthvalve 53 are opened. The remaining selected liquid 37, aerosol droplets39, rinse liquid 27, and any vapor from the rinse liquid and selectedliquid are then removed from the chamber 11 through the fourth port 51.This portion of the process may require another 10-20 sec. but may becontinued for a longer time interval, if desired, to completely removethe remaining selected liquid 37 and any remaining rinse liquid 27 fromthe films 16A, 16B, 16C and from the chamber 11. Drying of the objects13A, 13B, 13C is now substantially complete.

Optionally, hot dry nitrogen N2, carbon monoxide CO, carbon dioxide CO₂or other inert gas may be admitted into the chamber 11 through a fifthport 71 and associated fifth valve 73 to purge the chamber 11 and/orclean any remaining substances from the exposed surfaces 14A, 14B, 14Cof the objects 13A, 13B, 13C. The hot purge gas is received by thechamber 11 from a purge gas tank 75 and is removed through a sixth port81 and associated sixth valve 83 that may coincide with the fifth port71 and fifth valve 73, respectively. The hot purge gas is received fromthe chamber 11 in a spent purge gas tank 85 for recycling, processing ordisposal. This portion of the process, if included, may require another30-60 sec.

FIG. 4 is a flow chart indicating the process steps to be taken in oneembodiment of the invention. In step 91, the objects 13A, 13B, 13C(FIGS. 1 and 3) to be dried are placed into the chamber, and the chamberis closed. In step 93, rinse liquid 27 is admitted into the chamber topartially or (preferably) fully submerge the objects. In step 95,aerosol droplets of the selected liquid 37 are formed within thechamber, and a film of the selected liquid is formed and maintained onthe exposed surface of the tinge liquid. In step 97, the rinse liquid 27and any absorbed selected liquid 37 are slowly drained from the chamber,to ultimately expose the surfaces of the objects to the aerosol dropletsand to allow films of the selected liquid to form on the objectssurfaces. In step 99, the films of selected liquid on the objects'surfaces perform chemically squeegeeing to remove any remaining rinseliquid 27 and remaining selected liquid 37 from the objects' surfaces.In step 101 (optional), any remaining selected liquid 37 and rinseliquid 27 are removed from the chamber. In step 103 (optional), a hotpurge gas is passed through the chamber to remove any remaining gasand/or liquid particles from the chamber. The now-dried objects can nowbe removed from the chamber or may be further processed in the chamber.

I claim:
 1. Apparatus for drying an object, the apparatus comprising:anenclosed chamber, having interior chamber walls, for receiving andcontaining a selected object to be dried, the chamber having an openableentryway for allowing the selected object to be placed into, and to beremoved from, the enclosed chamber; a first entrance port for receivinga rinse liquid into the enclosed chamber, to partly or fully immerse theselected object in the rinse liquid; an second port for allowing therinse liquid to exit from the enclosed chamber at a first selectedvolume flow rate r1; a sonically vibrating head, located within theenclosed chamber, for receiving a selected volatile liquid, that doesnot chemically react with the selected object and that has a surfacetension the is substantially lower than the surface tension of the rinseliquid, and for vibrating to cause the selected liquid to form aerosoldroplets within the enclosed chamber; and a third port, connected to thevibrating head, to receive the selected liquid at a second selectedvolume flow rate r2 and to pass the selected liquid to the vibratinghead for dispersal within the enclosed chamber.
 2. The apparatus ofclaim 1, wherein said vibrating head vibrates at at least one frequencyf that lies in the range 10 kHz≦f≦1000 kHz.
 3. The apparatus of claim 2,wherein said frequency f is chosen so that at least one of said aerosoldroplets has a diameter d lying in the range 10 μm≦d≦50 μm.
 4. Theapparatus of claim 1, wherein said vibrating head vibrates at at leastone frequency f that lies in the range 20 kHz≦f≦100 kHz.
 5. Theapparatus of claim 1, wherein said selected liquid is drawn from a classof chemically substantially non-reactive liquids consisting of isopropylalcohol, ethyl alcohol, methyl alcohol, tetrahydrofuran, acetone,perfluorohexane, hexane and ether.
 6. The apparatus of claim 1, whereinsaid rate r1 is chosen so that said rinse liquid in said enclosedchamber has a depth that decreases at a rate of between 3 mm/sec and 10mm/sec.
 7. The apparatus of claim 1, wherein said rate r2 lies in arange 1 ml/min≦r2≦5 ml/min.
 8. The apparatus of claim 1, wherein saidsecond port delivers said selected liquid within said enclosed chamberat a pressure p lying in a range 5 psi≦p≦50 psi.
 9. The apparatus ofclaim 1, further comprising a source of said selected liquid, connectedto said third port and located outside said enclosed chamber.
 10. Theapparatus of claim 9, wherein said source of said selected liquid islocated at a distance of least one meter from said enclosed chamber. 11.The apparatus of claim 9, wherein said source of said selected liquidholds a volume V of said selected liquid that is no greater than 25 ml.12. The apparatus of claim 1, further comprising a fourth port forallowing said selected liquid and said aerosol droplets to exit fromsaid enclosed chamber.
 13. The apparatus of claim 1, further comprisingdroplet dispersal means, located within said enclosed chamber and nearsaid vibrating head and nozzle, for dispersing said aerosol droplets assaid aerosol droplets move away from said vibrating head and nozzle. 14.The apparatus of claim 12, wherein said droplet dispersal means includesa source of pressurized gas, maintained at a temperature T lying in therange 20° C.≦T-100° C., and a means for directing the pressurized gas sothat said aerosol droplets change direction as said aerosol dropletsmove away from said vibrating head and nozzle.
 15. The apparatus ofclaim 1, wherein substantially all of said aerosol droplets are formedwithin said enclosed chamber without a change of phase of said selectedliquid.
 16. The apparatus of claim 1, wherein said aerosol droplets areformed within said enclosed chamber with an energy expenditure of lessthan 2 Joules per second.
 17. The apparatus of claim 1, furthercomprising;a source of heated, relatively inert gas that can be used topurge said enclosed chamber; a fourth port, connected to the heatedinert gas source and attached to said enclosed chamber, to admit theheated inert gas into said enclosed chamber; and a fifth port, which maycoincide with the fourth port, to allow the heated inert gas to exitfrom said enclosed chamber.